Experimental study on suppressing heat transfer deterioration of supercritical CO2 in vertical tubes based on Helmholtz oscillator

Abstract

Heat transfer deterioration of supercritical CO2 (S-CO2) heated in gas heaters influences the efficiency and safe operation of the trans-critical CO2 system due to local high temperatures. To suppress the heat transfer deterioration of S-CO2, three Helmholtz oscillators with different nozzle diameters are designed and applied to the inlet of the heated vertical tube. The heat transfer characteristics of S-CO2 heated in the smooth tube and the smooth tube with Helmholtz oscillators are investigated at the mass fluxes from 144 to 750 kg/(m2·s), the heat fluxes from 23.0 to 128.1 kW/m2, the inlet temperatures of 15.0 and 25.0 °C, and the fixed pressure of 8.0 MPa. The results show that the heat transfer deterioration observed in the smooth tube is significantly inhibited by the Helmholtz oscillator. At G ≥ 400 kg/(m2·s), the ratio of local heat transfer coefficient and the thermal performance evaluation factor can be up to 9.86 and 3.09, respectively. The pulsating flow generated by the Helmholtz oscillator strengthens the mutual disturbance between near-wall fluid and the core fluid, resulting in more uniform thermophysical properties of S-CO2 in the radial direction and a much weaker buoyancy effect. In addition, the influence of the downstream nozzle diameter on the heat transfer characteristics of S-CO2 is more significant than that of the upstream nozzle diameter in the present study. Finally, a new empirical heat transfer correlation of S-CO2 in the smooth tube with Helmholtz oscillator is proposed, which takes into account the effect of the thermophysical properties of CO2 and the structure parameters of the Helmholtz oscillators with an accuracy of ±20 %.